1. Field of the Invention
The present invention relates to a motor driving technique.
2. Description of the Related Art
In recent years, increase in the operation speed of personal computers and workstations has led to rapid increase in the operation speeds of computation LSIs (large Scale Integrated Circuit) such as CPUs (Central Processing Unit), DSPs (Digital Signal Processor), etc. Such LSIs have a problem in that an increase in the operation speed, i.e., an increase in clock frequency involves an increase in heat generation. The heat generation of the LSI leads to thermal runaway of the LSI itself, or affects its peripheral circuits, which becomes a problem. Accordingly, such a situation requires a suitable thermal cooling operation for the LSI, as a crucial technique.
Examples of techniques for cooling an LSI includes an air-cooling cooling method employing a cooling fan. In this method, for example, a cooling fan is arranged such that it faces the surface of the LSI, and cool air is blown onto the surface of the LSI using the cooling fan.
With such a motor that drives such a cooling fan, if the motor locks due to a foreign substance being stuck in the cooling fan, this leads to an excessively large current flowing through a coil or a semiconductor element, which can damage the reliability of the device. In order to solve such a problem, a lock protection circuit is employed, which suspends the supply of electric power to the motor coil when the rotation of the motor stops.
In order to appropriately perform lock protection, there is a need to appropriately judge the motor rotation state. In a case in which the motor has a sensor such as a Hall element, such an arrangement is capable of judging in a sure manner based on the output signal from the sensor whether or not the motor is rotating (Patent document 1).
In contrast, in a case of employing a sensorless motor, such an arrangement cannot use the output signal from such a Hall element. In this case, such an arrangement requires an alternative method. Specifically, with such an arrangement, there is a need to judge the motor lock state based on the voltage (back electromotive force) that occurs across the motor coil.
As a technique for detecting the lock state of a sensorless motor, the present inventor has investigated the following technique (which will be referred to the “comparison technique”). A back electromotive force having a sine waveform occurs across the motor coil according to the rotation of the motor. With the comparison technique, a back electromotive force that occurs across at least one of the coils of the motor is compared with a common voltage of the coil by means of a comparator. When the motor is stably rotating, the back electromotive force thus generated crosses the common voltage in a cyclic manner. Thus, the output signal of the comparator (which will also be referred to as the “back electromotive force detection signal”) transits in a cyclic manner. Conversely, if the motor locks, the back electromotive force is not generated. Accordingly, the back electromotive force detection signal is not generated. Thus, with such a comparison technique, a motor lock state can be detected by monitoring the back electromotive force. Specifically, when the back electromotive force detection signal is not asserted, judgment is made that a motor lock state has occurred. Related techniques are described in Japanese Patent Application Laid Open No. 2005-6405, and Japanese Patent Application Laid-Open No. H10-234130.
Examples of control methods widely employed for a three-phase brushless motor include a 120-degree conduction system and a 180-degree conduction system (sine wave driving). The 120-degree conduction system has an advantage of allowing the control operation to be performed in a simple manner. As a tradeoff problem, such a 120-degree conduction system has a disadvantage in that noise and vibration are likely to occur as compared with the 180-degree conduction system. In contrast, the 180-degree conduction system has an advantage of low noise and low vibration, and an advantage of high efficiency. However, once the control operation goes out of synchronization, the system goes out of control. In order to solve such a problem, the 180-degree conduction system requires a complicated control method, as compared with the 120-degree conduction system.